Osteoclasts, the cells that resorb bone, are hematopoietic in origin and like other hematopoietic lineages arise from pluripotential stem cells, and mature through a series of developmental stages. With the exception of the terminal differentiation step to become mature bone resorbing cells, this developmental pathway(s) is poorly understood. This is particularly true of the early developmental stages. B lymphocytes (B cells), the cells which make antibodies, express cells surface molecules such as RANKL, which are involved in osteoclast differentiation. Loss of skeletal homeostasis can result in altered B cell function. We have begun an analysis of mice deficient in Pax5, a transcription factor required for the differentiation fo B cells. These animals experience a developmental block in B cell differentiation resulting in a phenotype characterized by the complete lack of mature B cells. Preliminary data indicate these mice are severely runted, develop strikingly decreased trabecular bone, with increased numbers of osteoclast precursors and osteoclasts, increased bone resorption, and reduced numbers of osteoblasts. It is our hypothesis that the loss of Pax5 leads to the deregulation of certain genes that enhance osteoclastogenesis and this, in turn, produces the resultant bone phenotype. This deregulation may also be responsible for the concomitant loss of B-cell lineage commitment. Three Specific Aims will be pursued: 1) Quantitative analysis of the bone phenotype in Pax5 deficient mice; 2) Quantitative functional analysis of mutant osteoblasts, stromal cells, and chondrocytes in vitro; and 3) Isolation and quantitative functional analysis of the osteoclast precursors. The long-term goal of this proposal is to identify the mechanism(s) by which Pax5 regulates normal osteoclast differentiation. Pax5 deficiency appears to be a novel model for osteoclast development and should allow for a detailed examination of osteoclast precursor development previously unobtainable. New models of osteoclast development present the potential to discover new, unrecognized catabolic pathways. This is particularly true for in vivo models with an established bone phenotype. Such information would be applicable to a wide variety of skeletal defects including, post-menopausal osteoporosis, age-related osteopenia, fracture repair, and extended survival of prosthetic implants.